Highly Selective H2S Gas Sensor Based on Ti3C2Tx MXene–Organic Composites

Cost-effective and high-performance H2S sensors are required for human health and environmental monitoring. 2D transition-metal carbides and nitrides (MXenes) are appealing candidates for gas sensing due to good conductivity and abundant surface functional groups but have been studied primarily for detecting NH3 and VOCs, with generally positive responses that are not highly selective to the target gases. Here, we report on a negative response of pristine Ti3C2Tx thin films for H2S gas sensing (in contrast to the other tested gases) and further optimization of the sensor performance using a composite of Ti3C2Tx flakes and conjugated polymers (poly[3,6-diamino-10-methylacridinium chloride-co-3,6-diaminoacridine-squaraine], PDS-Cl) with polar charged nitrogen. The composite, preserving the high selectivity of pristine Ti3C2Tx, exhibits an H2S sensing response of 2% at 5 ppm (a thirtyfold sensing enhancement) and a low limit of detection of 500 ppb. In addition, our density functional theory calculations indicate that the mixture of MXene surface functional groups needs to be taken into account to describe the sensing mechanism and the selectivity of the sensor in agreement with the experimental results. Thus, this report extends the application range of MXene-based composites to H2S sensors and deepens the understanding of their gas sensing mechanisms.


INTRODUCTION
Hydrogen sulfide (H 2 S) is a toxic and flammable gas, largely found in petroleum and mining industries as well as in our daily life, e.g., putridity of foods and bacterial breakdown of human and animal wastes. Exposure to H 2 S for humans has the risk of causing severe health problems such as eye and throat injury, dizziness, and loss of sense of reasoning at low concentrations, and it can even lead to death at a very high concentration (above 1000 ppm). According to the UK Health and Safety Executive standard, the short-term (about 8 h) exposure limit is 5 ppm. 1 Hence, the quality, performance, and accuracy of the detection sensors are very crucial. Most of the available H 2 S gas sensors, however, are expensive and suffer from various problems such as high cost and power consumption, the high limit of detection (LOD), low selectivity, and inflexibility. Therefore, developing a readily available and low-cost gas sensor with better selectivity and LOD toward H 2 S is necessary for human health and environmental monitoring.
Recently, two-dimensional (2D) materials (e.g., graphene, 2 MoS 2 , 3 black phosphorus, 4 and MXene 5 ) have attracted intensive research interest due to their unique physical and chemical properties, such as the large surface area, versatile surface chemistry, and room-temperature gas sensing capability. 6 Especially MXenes, consisting of 2D transition-metal carbides and nitrides, have shown promise for gas sensors due to outstanding metallic conductivity (10 3 −10 4 S cm −1 ), high mechanical stability, high hydrophilicity, and abundant surface chemistry for gas adsorption. 6 These layered materials have a universal formula of M n+1 X n T x , where M stands for early transition metals (Ti, V, Nb, Ta, Cr, Mo, etc.), X represents carbon and/or nitrogen, T x denotes the hydrophilic surface functional groups, such as �O, −OH, or −F, and n = 1−3. 7, 8 Lee et al. and Kim et al. were among the first to investigate gas sensing performance of pristine MXenes, 9,10 followed by a rapidly increasing number of studies. 11−15 In particular, the surface functional groups of MXenes provide a hydrophilic surface with highly negative zeta potentials, in the range of −30 to −80 mV, which facilitates efficient processing of hybrid MXene structures with organic polymers (with charged endgroups) in aqueous environment in contrast to other 2D materials. Forming composites of MXenes with organic materials has been already shown to be a feasible strategy to enhance both sensitivity and selectivity of gas sensors. 16−18 For instance, PEDOT:PSS/MXenes 6 and cationic polyacrylamide/ MXene composites 19 showed a high response to NH 3 , 36% at 100 ppm and 40% at 2000 ppm, respectively. However, despite the promising results, MXene-based gas sensors are still in their infancy and limited to sensors with a small response, constrained detection diversity with usually poor selectivity to the target gas. For example, to the best of our knowledge, only one H 2 S gas sensor and one electrochemical H 2 S sensor based on the Ti 3 C 2 T x -related materials have been reported thus far 20,21 in which the sensing response for the gas sensor was mainly attributed to the Ag nanoparticles. Moreover, the intriguing role of intrinsic surface functional groups in the gas sensing performance has not been evaluated extensively even with theoretical calculations, which impairs the understanding of the sensing mechanism.
In this work, we investigate the gas-sensing performance of pristine Ti 3 C 2 T x and its nanocomposites with poly[3,6diamino-10-methylacridinium chloride-co-3,6-diaminoacridine-squaraine] (PDS-Cl). While we observe clear H 2 S selectivity (negative response) already on the pristine thin film of Ti 3 C 2 T x sensors, the composites of PDS-Cl polymer and Ti 3 C 2 T x (Ti 3 C 2 T x /PDS-Cl) retain excellent selectivity toward H 2 S and provide a higher surface to volume ratio for MXene flakes that consequently enhances the sensing response (∼30 times higher compared to pristine MXene at 1 ppm H 2 S) with low detection limit (0.5 ppm) and good repeatability. To gain detailed insights into the interaction between gas molecules and Ti 3 C 2 T x , we carried out density-functional theory (DFT) calculations, where we accounted for the fact that MXene surfaces contain a mixture of �O, −OH, and −F functional groups. We show that this has a dramatic effect on gas adsorption (charge transfer and adsorption energy) and is necessary for reproducing the experimental observations. Based on these, we finally propose a sensing mechanism.

Synthesis of Ti 3 C 2 T x .
Aqueous dispersion of Ti 3 C 2 T x was synthesized using the MILD method with minor modification. 22 In a typical synthesis, Ti 3 AlC 2 (2 g, 325 mesh, Carbon-Ukraine) was added gradually to a stirring mixture of 40 mL 9 M HCl and 2 g LiF (2 g, 325 mesh, Sigma-Aldrich) at 35°C. After 24 h, the product was separated by a centrifuge and washed with deionized water (DI) until pH > 5. 40 mL water was then added to the sediment and vortexed for 30 min. The supernatant containing few and multiple layered Ti 3 C 2 T x was obtained by centrifuging the mixture at 3500 rpm for 15 min and then storing at 4°C before use. The concentration was measured by weighing a vacuum dried self-standing film of certain volume of the Ti 3 C 2 T x dispersion.

Synthesis of PDS-Cl.
Acriflavine (230 mg, Sigma-Aldrich) and squaric acid (114 mg, Sigma-Aldrich) were dissolved in 15 mL pyridine and 35 mL n-butanol, respectively. The two solutions were mixed and then refluxed and stirred at 120°C for 16 h under N 2 protection. After being cooled to room temperature, the mixture was filtered and washed using CH 2 Cl 2 , CH 3 OH, and saturated NaCl aqueous solution. The obtained PDS-Cl was dried in an oven at 80°C for 24 h. The product was collected as a dark brown powder.

Ti 3 C 2 T x /PDS-Cl Composite Preparation.
We synthesized poly[3,6-diamino-10-methylacridinium chloride-co-3,6-diaminoacridine-squaraine] (PDS-Cl), composited with Ti 3 C 2 T x through a facile in situ physical blending, as shown in Figure 1a where w Ti C T x 3 2 and w PDS-Cl are the weights of the Ti 3 C 2 T x and PDS-Cl, respectively. Table S1 shows the required mass ratio of Ti 3 C 2 T x and PDS-Cl for samples with different wt % of Ti 3 C 2 T x .
For instance, to prepare the Ti 3 C 2 T x /PDS-Cl with a mass ratio of 4 wt %, 1 mg PDS-Cl was dispersed in 1 mL DI water by bath sonication for 30 min. After that, 3.3 μL of Ti 3 C 2 T x aqueous ink (C = 12 mg•mL −1 ) was added to the solution followed by 2 min of tip sonication at 20 W to achieve uniform distribution of Ti 3 C 2 T x . The same procedure was repeated for other mass ratios by changing the volume of Ti 3 C 2 T x aqueous ink.
2.4. Characterization. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Thermo Fisher Scientific Escalab 250 XI system with an Al Kα source. Raman spectra were performed by a Thermo Scientific DXR2xi Raman imaging microscope (excitation wavelength, λ = 785 nm). The microstructure of synthesized material was studied by field-emission scanning electron microscopy (FESEM, Zeiss ULTRA plus and equipped with EDX), transmission electron microscopy (TEM, JEOL JEM-2200FS EFTEM/STEM 200 kV), and energy-dispersive X-ray elemental mapping. Fourier transform infrared spectroscopy (FTIR) of MXene−polymer hybrid was performed on a Spectrum Two FT-IR spectrometer with an ATR model (PerkinElmer, UK). The X-ray diffraction (XRD) was carried out by Rigaku Smart Lab 9 kW, Cu Kαradiation with a 0.02°of step width. Dynamic light scattering (DLS) data were obtained using a Zeta sizer Nano ZS 90.
2.5. Sensor Fabrication and Gas Sensing Setup. To fabricate sensors, 1 μL DI dispersion (1 mg•mL −1 ) of the Ti 3 C 2 T x /PDS-Cl composite was drop-casted on the Si/SiO 2 substrate (4 × 6 × 0.5 mm), with 25 pairs of Au−Ti interdigitated electrodes (electrode distance and width were both 20 μm), to form a sensitive film and dried at room temperature, as shown in Figure 1c.
The sensing performance of materials was studied in a Linkam THMS600 heating and freezing stage connected to an Agilent 3458A multimeter at 1 V of constant bias, which are shown in Figure S1. Different concentrations of NH 3 , NO, H 2 S, CH 4 , CO, and H 2 were obtained by LabView driven mass flow controllers. Nitrogen gas (N 2 ) and dry air were used as the carrier gas to dilute these gases to the desired concentrations, while the operating temperature was maintained at room temperature (30°C). The total gas flow rate was kept constant at 100 mL min −1 in all experiments except for selectivity that it was 500 mL min −1 . To adjust the relative humidity level, an airflow was bubbled through a water-containing flask and then diluted with N 2 gas before introducing it into the test chamber. The ultimate humidity of the test gas was calibrated via a commercial humidity sensor.
2.6. Computational Methods. All DFT calculations were performed using cp2k software. 23−25 The PBEsol functional 25 was used with a Goedecker−Teter−Hutter (GTH) 26,27 pseudopotential and the Gaussian-type basis set MOLOPT. 28 Van der Waals interactions were accounted by adding the D3 dispersion correction. 29 A cutoff energy of 800 Ry was set for the expansion of the electron density in plane waves. Geometry optimizations were performed on 4 × 4 × 1 supercell using a 3 × 3 × 1 Monkhorst−Pack k-point grid. An out-of-plane lattice constant of 30 Å was used to provide a sufficiently large vacuum region (MXene thickness as calculated from the outermost H atoms is about 9.2 Å). The other lattice parameters were optimized for both mixed and fully O-terminated surfaces and the optimal values were then used and kept constant when adding the gas molecules. The charge transfer was computed using the Mulliken charges as directly implemented in cp2k. We defined it as the difference between the total MXene charge in the presence of and without the gas molecule. This means that a positive charge transfer represents electrons going from the molecule to the MXene layer. The adsorption energy E a was defined as ads MXene mol , where E ads , E MXene , and E mol are the energies of the MXene layer with the adsorbed molecule, the pristine MXene layer, and the isolated gas molecule, respectively. The adsorption energy and charge transfer calculated without the D3 dispersion correction are listed in Table S2.

Material Characterization.
We carried out material characterization for both pristine Ti 3 C 2 T x and its polymer nanocomposites. The polar charged structure of squaraine in PDS-Cl provides stronger electrostatic interactions between polymers and the Ti 3 C 2 T x flakes, 30 and it can offer ion−dipole interactions or hydrogen bonding with MXenes, forming numerous heterojunction interfaces that may benefit the gassensing properties of the composite. For instance, DLS data in Figure S2 show that increasing the MXene content gradually increases the average particle size from 371 nm in the pristine MXene to 4317 nm in PM-20. Such soft aggregation is caused by the electrostatic interaction between negatively charged MXene flakes and the positively charged quaternary ammonium cations in the PDS-Cl. 31 Scanning electron microscopy (SEM) images of the composite, as shown in Figure 2a,b, show the heterojunction interfaces and the layered structures of Ti 3 C 2 T x flakes. This layered structure is also evident from the TEM analysis of pristine MXene, as shown in Figure 2c. TEM images of pristine Ti 3 C 2 T x and the composites (Figure 2c,d, respectively) indicate a successful blending of the MXene and polymer even for very thin samples, which is further supported by energy-dispersive X-ray (EDX) imaging that provides the elemental distribution of the composite structure, as shown in Figure S3. Pristine MXene sheets should be free of N and Cl but present in the polymer, whereas the opposite is true for Ti. Moreover, EDX indicates the presence of fluorine atoms that probably originate from the functional groups at the surface of the Ti 3 C 2 T x flakes.
Fourier transform infrared (FTIR) and Raman spectra of pristine Ti 3 C 2 T x , PDS-Cl polymers, and the Ti 3 C 2 T x /PDS-Cl composite, as shown in Figure 3, reveal the ionic nature of the PDS-Cl and the hydrogen bonding between the MXene flakes and the polymers. For a better comparison of FTIR and Raman spectra, we have only selected 3 composite samples (out of 6 samples utilized for gas sensing) with mass ratios of 4, 10, and 20 wt % denoted as MP-4, MP-10, and MP-20, respectively. In the FTIR spectrum for the PDS-Cl, as shown in Figure 3a, the peaks at 3322 and 3187 cm −1 are attributed to the N−H group. The feature at 1783 cm −1 can be attributed to the cyclobutene carbonyl compound (C�O), 32 which overlaps with expected C�N (imino) resonances at ca. 1780 cm −1 . The peak at 1544 cm −1 originates from the C�C stretching vibrations of four-membered ring while the characteristic strong absorption peak at 1610 cm −1 in PDS-Cl can be assigned to the vibration peak of the aromatic structure and zwitterionic resonance of the cyclobutene 1,3-diolate anion moiety, indicating the successful synthesis of the PDS-Cl with the anticipated structure, as shown in Figure 1a. The FTIR   and C−N + (at 1459 and 1390 cm −1 , respectively) imply the ionic nature of PDS-Cl. 37,38 Although the FTIR spectrum of pristine Ti 3 C 2 T x exhibits few characteristic features, as shown in Figure 3a, the Raman spectrum of thin films of Ti 3 C 2 T x and MP-10, dispersed on a Si substrate, indicates several features, as shown in Figure S4. Spectra show peaks at 123, 202, and 723 cm − , which are assigned to the plasmonic resonance, the out-of-plane vibration of Ti, C, and surface group atoms, A 1g (Ti, C, and O), and the out-of-plane vibration of carbon atoms, A 1g (C), respectively. The intensity ratio of A 1g (C)/A 1g (Ti, C, O) varies depending on the sample. 39 For instance, this ratio for the pristine sample is around 0.86, which implies a strong A 1g (Ti, C, and O) vibration as whole flakes while for composites, it increases to 1.2 as a sign of weak coupling between flakes due to intercalation of polymers. Moreover, the A 1g (Ti, C, and O) peak for composite shifts to lower wavenumbers, ∼198 cm −1 , further indicating the disorder between the layers. 39 XRD measurements also support the disorder between the Ti 3 C 2 T x flakes in the composites, as shown in Figure 4a. The characteristic reflection of the pristine Ti 3 C 2 T x at 8°(11.0 Å) shifts to 6.4°(13.8 Å) for MP-10 and MP-20 along with peak broadening and disappears for the MP-4 sample, implying increased interlayer separation and a disorder in the stacking of the Ti 3 C 2 T x flakes due to intercalation of PDS-Cl between layers. 40 The functional groups, such as fluorine and oxygen, at the surface of the Ti 3 C 2 T x flakes and composite samples were evaluated using XPS. Figure 4b shows the survey spectrum of Ti 3 C 2 T x , PDS-Cl, and MP-10, indicating the presence of those functional groups on the surface of the composite. The C 1s and F 1s spectra in Figure S5 indicate the interaction of carbon and fluorine in the MP-10 sample. 41 In the F 1s spectrum of the pristine MXene ( Figure S5b), the peaks at 684.5 and 685.2 eV are assigned to F−Ti and C−Ti−F x , respectively, 42,43 whereas the F 1s spectrum for MP-10 contains a third peak at 689.65 eV, suggesting the presence of the interaction between carbon and fluorine with high binding energy.
3.2. Gas Sensing Performance. The gas sensors were fabricated by drop-casting equal amounts (1 μL of solution with concentration of 1 mg•mL −1 ) but different mass ratios of the MXene/PDS-Cl composite onto SiO 2 (300 nm)-Si substrates. To find out the optimum mass ratio with the highest sensing response, we measured the H 2 S response of composite samples with 6 different mass ratios of 4, 6, 8, 10, 15, and 20 wt %. Figure S6 shows the base resistance of the Ti 3 C 2 T x /PDS-Cl composite samples, prior to gas sensing. A percolation threshold of φ 0 = 6.00 ± 1.10 wt %, obtained by Belehradek power function fitting, indicates that all samples, other than 4%, contain conductive networks of MXene flakes.
The gas response is calculated using eq 2 where R 0 andR g are the resistances of the sensor upon exposure to N 2 and the target gas, respectively. Figure 5a summarizes the selectivity responses from the MP-10 sample, and the results from pristine MXenes, in the inset. The composite sensor shows striking selectivity toward the H 2 S gas, with a negative response, while it is positive for all other analytes (see Figure S7 for real-time resistance curve of the sensor). Notably, the selectivity to H 2 S is already present in the pristine MXene sensor, although the response is much smaller as shown in the inset of Figure 5a and Figure S8a.
Among all the samples, MP-10 shows the highest sensing response, around 2 ± 0.2% at 5 ppm of H 2 S gas, as shown in Figure 5b, with a 30-fold increase of response at 1 ppm concentration of H 2 S compared to the pristine Ti 3 C 2 T x (see Figure S8b). However, the noise level in MP-10 is higher compared to pristine Ti 3 C 2 T x in Figure S8b because the polymer alters the charge transport between layers. The realtime resistance curves of gas sensing for all samples (with different wt % of Ti 3 C 2 T x ) are shown in Figure S9. Figure 5c shows the dynamic sensing response of the sensor for different concentrations of H 2 S where the baseline is subtracted for clarity and readability of the signal. Despite the long pre-measurement exposure to N 2 , around 1 h, the baseline is drifting; therefore, to calculate the correct response, a baseline curve, R 0 has been fitted and subtracted from the data. The sensor response is a linear function of the gas concentration with a low LOD as low as 0.5 ppm, see Figure  5d. To ensure the result for LOD, we took 40 data points at the baseline before the H 2 S exposure in Figure 5c and calculated the noise level using the variation in the relative resistance change by the root-mean-square deviation. 44 The absolute value of the response for MP-10 (0.14%) is at least five times higher than the noise level (0.023%), confirming a LOD of 0.5 ppm. Finally, we note that the sensor response toward humidity was also positive, as shown in Figure S10, with enhanced responsivity, compared to pristine Ti 3 C 2 T x .
The sensor shows very good repeatability with a small variation (standard deviation of 0.076) under consecutive exposure to the H 2 S gas and recovers back to its initial state after gas removal, as demonstrated in Figure 6a.
The sensor stability under continuous measurements indicates that the response drops from −2.2 to −1.25% for 5 ppm of H 2 S after 10 days, as shown in Figure 6b, indicating a 60% stability compared to the initial response. Figure 6c,d shows the sensor response to H 2 S with concentrations varying from 0.5 to 5 ppm at day 1 and 10, respectively. The stability data is noisy, which could originate from the measurement setup rather than the material because exposure to the H 2 S gas has no effect on the noise level. To improve the readability, the data have been smoothed with 10 points adjacent averaging method. Figure S11 demonstrates the real-time resistance curve of the sensor for H 2 S sensing on days 1 and 10, in which the base resistance of the sensor increases in the course of time. Further studies are required to understand and enhance the stability, which might originate from the Ti 3 C 2 T x oxidation and can be suppressed by modifying the device preparation or structure. 13 In order to test the sensor's performance for realworld applications, we also measured the H 2 S sensing performance of the MP-10 in an N 2 /O 2 (80/20%) background, as shown in Figure S12, which indicates almost identical performance of the sensor.

H 2 S Gas Sensing Mechanism.
The pronounced H 2 S selectivity may seem surprising in light of the previous results reported in the literature showing a positive response to all analytes. Pristine Ti 3 C 2 T x films were studied in refs 91045, and 48 and the general trends in the sensitivity agree with our results: sensitivity to ammonia and other "reactive" gases was high, but low to gases such as CH 4 and CO 2 . In refs 10 and 45, all gases showed a positive response, but H 2 S was not included in these studies. Wu et al. measured the H 2 S response, but they only reported |ΔR|/R, i.e., the sign of the response is unknown, and the gas concentration was very high (500 ppm). 45 Thus, although most papers have reported a positive response to any gas, the understanding of H 2 S response is, in fact, limited.
Using in situ XRD, Koh et al. found that the positive response of "reactive" gases such as ethanol correlated with increasing interlayer separation of (Na-intercalated) Ti 3 C 2 . 46 This suggests a sensing mechanism, where the gases are intercalated between the layers and the resistivity increases due to the increasing interlayer separation of the conductive MXene sheets. Such a mechanism could explain the response to all gases with a positive response and the enhanced gas response in the MXene/polymer composite. However, the negative response (increasing conductivity) to H 2 S requires an inherently different mechanism. Also, the linear I−V characteristics of MP-10 ( Figure S13) suggest ohmic contact between the sensing film and the electrodes; therefore, the H 2 S exposure modulates the conductivity of composites rather than contact resistance.
To gain insights into the possible sensing mechanism, we turned to atomistic modeling and DFT calculations. Because the negative response for H 2 S was recorded already for pristine Ti 3 C 2 T x samples, we are looking for a mechanism that does not require the polymer but can still be enhanced by it. The majority of previously reported calculations have only considered pure O-terminated or pure OH-terminated surface. 10,45,47,48 However, it is known from NMR and neutron/ X-ray scattering experiments that the surfaces contain a mixture of O, OH, and F groups, 49,50 and this functionalization is stable in vacuum or N 2 atmosphere. 51 In order to properly describe the interaction between the gas molecule and MXene surface, we thus adopt a model which contains a mixture of O, OH, and F groups in the O 0.50 OH 0.25 F 0.25 composition. The adopted model was found based on our previous investigation, reflecting typically reported compositions. 52 A significant concentration of O and F in our samples was also verified by EDX and XPS (Figures 2c and 4), although we cannot estimate the H concentration (i.e., the O/OH ratio) based on these methods. We note that gas adsorption on mixed-group surfaces of Ti 3 C 2 T x was studied by Khakbaz et al., 53 but (i) these results were not compared to those from pure terminations and (ii) there was no detailed comparison to experimental results.
The charge transfers and adsorption energies of various gases on the pure O-terminated surface and the mixed-group surface are given in Table 1 and the adsorption geometries are depicted in Figure 7. The results are not only quantitatively but also qualitatively different owing to the larger variety of possible adsorption sites on the mixed surface and the different work functions (comparison of adsorption energies and charge transfer for similar sites are given in Table S2). We particularly note that H 2 S and H 2 O bind very strongly to mixed surfaces (much stronger than to the pure O-terminated surface) because the S atom of H 2 S and O atom of H 2 O can bind to the OH group and the H atoms to O groups. In fact, this result is fully consistent with reports of interlayer and surface water in MXenes (as observed, e.g., in TGA experiments), 54 arising from its high hydrophilicity. 55 NH 3 appears to have even stronger adsorption energy, although this situation is somewhat different. Because when NH 3 is adsorbed on the mixed surface, it captures a proton from one of the surface OHgroups (and 0.288e), resulting essentially in NH 4 + adsorbed on negatively charged MXenes. We note that similarly high adsorption energies and NH 4 + formation have been reported for mixed surfaces of vanadium carbide. 56 Such high adsorption energies would strongly favor analyte adsorption but rule out analyte desorption under ambient conditions, which is clearly not the case according to experiments. Instead, we propose that the omnipresent water will play a central role here in displacing the analytes from the surface, i.e., the analyte adsorption should rather be described by the adsorption energy difference with respect to H 2 O.
The markedly higher work function of the O-terminated surface (calculated to be 6.17 eV 52 vs 4.3 eV for the mixed surface adopted here, Figure S15) is expected to lead to larger electron transfer from the analyte to MXene, i.e., more positive values for charge transfer in Table 1. This is indeed the case of CH 4 and NO on the O-site and H 2 S on the O-site (Table S3). However, these are not the lowest energy configurations for NO and H 2 S, and when the analyte is bonded to OH groups the charge transfer can be qualitatively different.
In light of the above discussion, two mechanisms are likely at play: (1) the intercalation-induced increase of the interlayer separation and (2) charge transfer-induced modifications in carrier density. Mechanism 1 will contribute to all analytes, but it will dominate whenever the adsorption energy of the analyte  (Figures S7 and S14) also points to a strong interaction.
In the case of MXene/PDS-Cl composites, the response is enhanced while the selectivity is preserved. As the polymer opens the interlayer spaces, the number of accessible active sites increases and the intercalation becomes easier. This can lead to enhancement of the charge-transfer effect (mechanism 2). As for mechanism 1, the situation is less clear: a larger concentration of intercalated analytes between the sheets can enhance the effect, while the induced changes in the interlayer separation might be reduced due to initially larger spacing. A final contribution arises from the geometrical effect, wherein the number of conductive paths is reduced with increasing polymer content (or decreasing the wt % of Ti 3 C 2 T x ; see resistance in Figures S5 and S8), which makes the sensor more sensitive (and noisy). On the other hand, the number of adsorption sites is expected to increase with increasing polymer content, and therefore there should be a trade-off for response as Figure 5b indicates. As the PDS-Cl/CNT composite also exhibits a small negative response toward H 2 S whereas bare CNTs do not, see Figure S15. We cannot rule out a synergic effect based on the adsorption of analytes on the PDS-Cl. However, we note that the conductivity of bare PDS-Cl samples was below the detection limit of our instruments and according to our calculations, the MXene Fermi-level resides between the frontier orbitals of PDS-Cl ( Figure S16) consistent with small conductivity.

CONCLUSIONS
In summary, we have observed high selectivity in pristine thin films of Ti 3 C 2 T x toward H 2 S gas sensing. Utilizing conjugated PDS-Cl polymers, we could preserve the selectivity and enhance the gas-sensing response thirtyfold at 1 ppm. The optimized sensor with 10 wt % of MXenes indicated a response of 2% at 5 ppm with an LOD of 500 ppb. To shed light on the sensing mechanism, we carried out DFT calculations. We have accounted for the fact that MXene surfaces contain a mixture of O, OH, and F functional groups and show that this has a dramatic effect on the gas adsorption (charge transfer and adsorption energy). The experimentally observed trends could be reproduced relying on the analyte intercalation and the charge-transfer mechanism from adsorbed analytes in competition with water molecules. This report expands the MXene/organic heterojunction application and enhances the understanding of gas sensing mechanisms in MXene-based sensors.
■ ASSOCIATED CONTENT
Required mass ratio of Ti 3 C 2 T x and PDS-CL for samples with different wt % of Ti 3 C 2 T x ; schematic of gas sensing setup; DLS data of pristine MXene, MP-4, 10, and 20; EDX imaging of the composite sample; Raman spectra of pristine Ti 3 C 2 T x and MP-10; XPS spectra for C 1s and F 1s; base resistance of composite sample with different weight ratios of Ti 3 C 2 T x along with power function fitting; real-time resistance curve of MP-10 sensor for different analytes; selectivity and real-time resistance curve of pristine Ti 3 C 2 T x for different concentrations of H 2 S; I−V measurement of MP-10; real-time resistance curves; charge transfers and adsorption energies for the different molecules calculated with PBEsol functional but without van der Waals corrections; charge transfers and adsorption energies for the different molecules when adsorbed on the mixed surface and the purely O-terminated surface; and energy levels of the PDS-Cl polymer compared to the Fermi level of mixed MXenes (PDF) ■ AUTHOR INFORMATION